1. Field of the Invention
The present invention relates to signal processing circuits and systems. More specifically, the present invention relates to circuits and systems for providing automatic gain and offset control.
2. Description of the Related Art
Night vision systems are well known in the art. Night vision systems typically include a cryogenically cooled linear detector array with an associated cryogenic subsystem, a scanning system which moves an image across a two-dimensional array, and a refractive optical system which focuses energy onto the detector. The detectors in the array either sense the heat of a body or detect low light levels.
While these systems have been used for military applications, the high cost of the scanning and cooling of the array and the optical systems associated therewith has heretofore limited the applicability of same for numerous other applications. Accordingly, there has been a need in the art for a low cost night vision system.
U.S. patent application Ser. No. 08/232,893, entitled "LOW COST NIGHT VISION CAMERA" Apr. 12, 1994 by S. H. Klapper et al., the teachings of which are incorporated herein by reference, discloses and claims a low cost camera for night vision systems including a focal plane array of uncooled detectors and an optically fast, optical arrangement for focusing energy onto the array. The array may include a plurality of pyroelectric detectors which in the illustrative embodiment are fabricated of barium-strontium-titanate material. Each pixel in the array is associated with a unique detector.
Unfortunately, the characteristics and therefore the output of each detector is unique with respect to sensitivity, gain and DC offsets. U.S. patent application Ser. No. 08/226,796 filed Apr. 12, 1994, the teachings of which are incorporated herein by reference, discloses and claims a signal processing circuit which provides automatic gain and offset control for an array of BST detectors. U.S. patent application Ser. No. 08/226,588 filed Apr. 12, 1994, the teachings of which are incorporated herein by reference, discloses and claims a digital signal processing circuit which provides automatic offset and global gain control for an array of pyroelectric detectors.
These references provide a teaching as to how to correct for sensitivity and gain variations for individual detectors in the array. Correction of gain and sensitivity variation on a detector by detector basis is effected by exposing the array to a uniform warm reference pattern and storing the outputs of the detectors in memory. In operation, the stored values are converted to analog form by a digital-to-analog converter (DAC) and used to adjust a reference input for a multiplier. In operation, a signal from a given detector is multiplied by this unique stored gain correction parameter to effect the gain adjustment.
However, the range of gain adjustments required across one array may differ considerably from the range of adjustments required across another array. That is, gain correction across one array might vary from 0.5 to 1.5, while the gain correction variation across another array may vary from 0.9 to 1.1. Hence, one array may require the full dynamic range of the DAC, while another may require only a portion of same. This leads to an underutilization of the dynamic range of the DAC for certain arrays and an associated suboptimum performance. Digital-to-analog converters are typically expensive devices relative to other components in the system and these devices often set the dominant parameters of system performance. Full utilization of the dynamic range of a DAC allows a full utilization of the resolution of the device. (In this context, the term "resolution" means the extent to which the device can discriminate between input signals that are close in amplitude or digital code and provide corresponding output signals which are widely spaced.) Increases in correction resolution allow for the use of low cost digital-to-analog converters.
Accordingly, a need remains in the art for a system and technique for providing an optimum utilization of the full dynamic range of the an digital-to-analog converter.